Ehmm, Stonehenge is not actually in London
(If it was the land would be worth so much, they'd outsource Stonehenge to Bangalore and build an office building there instead)
I am not a big fan of the touristy stuff, I am more into just walking around the city, relaxing, getting a beer, seeing what real people actually do.
In London you can:
- See a west-end musical
- Walk around Soho and grab a pint, or 7
- Go to the Notting Hill market and to Camden Town, or maybe Brick Lane as well for some folklore.
- Take a walk in Hyde Park/Regent's Park.
- Go to the British Museum
- Go to Tate Modern if you're into modern art
- Walk around Chelesa, Knightsbridge, go to Harrods etc...
- See the usual sights: Tower Bridge, London Eye, Buckingham Palace, Tower of London etc (nothing overly exciting, but good to just see them)
- In general I suggest just taking a few days to walk around, and make regular stops in pubs.
- Do the walk around Champs Elysees, see the Eiffel Tower, Arc de Triomphe etc...
- Go for a walk around Montmartre (very nice)
- Go to the Louvre, or if you are actually interested in good art other than seeing the Mona Lisa just for the sake of seeing it, go to the Musee D'Orsay (a lot of good Impressionism) or Centre Pompidou (lots of good modern art)
- Visit the Quartier Latin
- Same thing as for London - just walk around, get a feel for the city, not worth it to run around like madman to see all the sights. Most Parisians probably haven't.
FYI: there is not much to do in Frankfurt. It's a nice city, but if you have seen Berlin or Munich it's probably not worth the effort. Unless you want to meet some ECB Central Bankers.
Milan, Venice, and Rome are all cool - worth visiting IMO, so is Berlin. I would suggest mabe trying to see Prague - very nice old town, cheaper than rest of Europe, and excellent beer (just don't come on the weekend - too many drunken Brits). Vienna could be interesting too, or Budapest.
edit: terrible spelling